1. Field of the Invention:
The present invention relates to an optical plane circuit comprising an optical waveguide formed in a transparent substrate and an optical coupler for improving input and/or output of a light beam into and/or from the optical waveguide, and to a method for manufacturing the same.
2. Description of the Prior Art:
In the fields of optical communication and optical information processing systems, an optical plane circuit wherein a small optical waveguide of a refractive index larger than that of a transparent substrate is formed on the surface or inside the substrate for achieving various functions is receiving a lot of attention as a small optical circuit. For example, when a glass plate is used as the substrate, a buried-type optical waveguide is known to be obtained which utilizes ion exchange. Since the transmission loss of the optical waveguide obtained in this manner is assumed to be as small as 0.01 dB/cm, this optical waveguide is considered promising. However, when ion exchange is utilized, the cross section of the optical waveguide is larger in the transverse direction than in the vertical direction. For this reason, when an optical fiber or the like is directly connected to the optical waveguide, the connection loss is increased.
Various methods have been proposed for input and/or output of light into and/or from an optical plane circuit. FIG. 1 shows a method which utilizes a prism coupler. In the method shown in FIG. 1, an optical waveguide 12 having a constant or graded refractive index is formed on the front surface of the substrate 11. Prisms 13 and 14 are arranged on the surface of the optical waveguide 12 with their bottoms facing it. A light beam 15 incident on the prism 13 then becomes incident, at the bottom surface of the prism 13, on the optical waveguide 12 at an angle .theta..sub.1 slightly larger than the total reflection angle and then propagates within the optical waveguide 12. When the light beam 15 reaches the bottom surface of the prism 14, the light beam 15 becomes incident on the prism 14 at an angle .theta..sub.2 and then emerges to the outside.
FIG. 2 shows another method for optical coupling which uses a focusing rod lens. In the method shown in FIG. 2, one end of a focusing rod lens 24 is adhered to a side surface 23 of an optical waveguide 21 which is buried in the surface layer of a substrate 22. The overall length of the focusing rod lens 24 is selected to be odd number times of 1/4 the periodical pitch of light. The other end 25 of the focusing-rod lens 24 is cut perpendicularly with respect to the central axis and optically polished. A light beam 26 becomes incident on this other end 25 perpendicularly thereto. The angle of incidence of the light beam 26 is controlled by a light output device (not shown) without bringing about changes in the position and size of the beam spot incident on the side surface of the optical waveguide.
However, the methods as shown in FIGS. 1 and 2 are subject to the drawbacks to be described below.
(a) Since the optical waveguide and the optical coupler are not formed in a single substrate, troublesome precision registration is required during assembly of these two optical circuit elements.
(b) The optical coupler is greater than the optical waveguide, so that the overall circuit cannot be made compact in size.
(c) Connection loss is caused due to connection between the optical waveguide and the optical coupler.
(d) If the cross section of the optical waveguide extends wide along the plane of the substrate, the coupling efficiency is degraded.